Preferential solid electrolyte interphase formation on graphite and post-oxidative cathode-electrolyte interphase refinement: A bifunctional sulfonate additive for durable graphite||LiCoO2 batteries.

To address interfacial instability at the graphite anode and high-voltage degradation of the LiCoO2 cathode in lithium-ion batteries, 2,2,2-trifluoroethyl methanesulfonate (TM) is proposed as a bifunctional film-forming electrolyte additive for graphite||LiCoO2 cells. Density functional theory calculations and electrochemical analyses reveal that TM exhibits a higher reduction tendency than carbonate solvents, enabling its preferential decomposition on the graphite surface. This process yields a thin, low-impedance, and Li+-conductive solid electrolyte interphase (SEI) enriched with LiF and sulfur-containing species, which effectively suppresses continuous electrolyte reduction. On the cathode side, although TM does not undergo preferential oxidation relative to the baseline electrolyte, it actively participates in the formation and stabilization of a uniform and compact cathode-electrolyte interphase (CEI). This TM-modulated CEI mitigates electrolyte oxidation, inhibits cobalt dissolution, and alleviates structural degradation of LiCoO2 under high-voltage operation. Benefiting from the synergistic stabilization of both electrode-electrolyte interfaces, Li||graphite half-cells incorporating 1 wt% TM retain a reversible capacity of 323.7 mAh·g-1 after 550 cycles, markedly outperforming the baseline system (104.4 mAh·g-1). In graphite||LiCoO2 pouch cells, the TM-containing electrolyte improves capacity retention from 31.21% to 45.75% over 200 cycles. Furthermore, at -10 °C, full cells with TM deliver 75.4 mAh·g-1, achieving approximately 40% higher capacity retention and significantly reduced polarization compared to the baseline. These findings establish TM as an effective dual-interface modifier and demonstrate a practical interphase engineering strategy for high-voltage and low-temperature lithium-ion batteries.